1. Field of the Invention
The present invention relates to an image capture apparatus, an image capture method, a computer program and a computer-readable recording medium for capturing, recording and reproducing a still image or a moving image, and more particularly to a technique for correcting a captured image.
2. Description of the Related Art
Image capture apparatuses, such as electronic cameras, that have been commercially available in recent years include an image capture apparatus that uses a memory card having a solid-state memory element as a recording medium. This type of image capture apparatus captures a still image or a moving image using a solid-state image sensor, such as a CCD (charge-coupled device) or a CMOS (complementary metal-oxide semiconductor) sensor, and records or reproduces the captured image on or from the memory card.
In capturing an image using a solid-state image sensor, such as a CCD or a CMOS sensor, the image capture apparatus is capable of performing dark noise correction. In the dark noise correction, an arithmetical operation is performed using dark image data and originally captured image data. The dark image data is read out from the image sensor after charge storage is performed in the same manner as in an exposure operation while the image sensor is not exposed to light. The originally captured image data is read out from the image sensor after charge storage is performed while the image sensor is exposed to light.
However, in the conventional image capture apparatus, a shutter release time lag is increased by a period of time required for obtaining dark image data, so that a user may miss an important photo opportunity.
Japanese Laid-Open Patent Application No. 2003-333434 discloses a horizontal dark shading correction method using one-dimensional correction data in order to provide an image capture apparatus, an image capture method, a program and a recording medium that are capable of preventing image quality deterioration by reducing a circuit system noise, i.e., a fixed pattern noise which is generated as dark offset by voltage fluctuations caused by the resistance component of a power line in an image sensor or variations of image sensors, while minimizing the number of times of obtaining dark image data.
When long exposure is performed, particularly, under the condition that an increase in a dark current component is not negligible, e.g., high temperature or high ISO sensitivity, it is necessary to limit an output voltage from an image sensor including a dark current to a range that does not exceed the dynamic range of an analog processing portion.
In order to solve the above problem, generally, the output level of an optical black portion of an image sensor is detected, and a clamping circuit using this level as black reference is employed.
Japanese Laid-Open Patent Application No. 2001-326850 discloses a method for solving the above problem by using an image capture system. In this image capture system, when an exposure time exceeds a predetermined time, readout from an image sensor is performed a plurality of times during a single exposure operation to obtain a plurality of video signals. The plurality of video signals are recorded on a recording medium. Then, a single video signal is created by performing an arithmetical operation on the recorded plurality of video signals.
However, the above-described image capture apparatus capable of performing horizontal dark shading correction has the following problem when long exposure is performed. Correction data used for horizontal dark shading correction is obtained as horizontal one-line correction data by performing dark image photography during the manufacturing process of image capture apparatuses and performing a projection operation on an image obtained by the dark image photography.
In performing dark shading correction using the correction data, readout is performed a plurality of times to secure dynamic range when long exposure is performed. When readout is performed a plurality of times, a circuit noise is generated every time readout is performed. Therefore, it is necessary to perform horizontal dark shading correction for every readout operation. Accordingly, with respect to image data that is finally obtained by an arithmetical operation using all of the read-out images, horizontal dark shading correction has to be performed in total a number of times corresponding to the number of times that readout is performed. Therefore, a shutter release time lag increases, so that a user may miss a photo opportunity. In addition, since quantization errors that occur during horizontal dark shading correction are accumulated, image quality may deteriorate in the case of a photography mode in which readout is performed a plurality of times.
FIGS. 10A and 10B illustrate horizontal dark shading correction that is performed as described above on dark image data. In the case of short exposure shown in FIG. 10A, correction is performed only once. Correction data in this case has a resolution of 1 LSB (least significant bit) value. Image data after dark noise correction (dark image) is data that has been adequately corrected with a precision of 1 LSB value or less.
In the case of long exposure shown in FIG. 10B, results of correction performed a plurality of times are added together. In this case, the following problem arises. Let n represent the number of times of correction. Then, image data can be considered to have been corrected using correction data having a resolution of the sum of n LSB values in total. An image signal obtained from the image sensor retains image information having a resolution of 1 LSB value even after the addition operation, although the image signal contains quantization errors and noise components.
However, if correction data used for the correction operation has a resolution of the sum of n LSB values (n=4 in the case of FIG. 10B), image data after dark noise correction becomes image data having only a precision of the sum of 4 LSB values. In this case, a discontinuous image results. For example, if dark shading correction in the horizontal direction is performed, an image having a vertically-striped pattern within an image frame results. Thus, the image quality of a captured image deteriorates.